The present invention relates generally to the field of sensor technology. More specifically, the present invention relates to the development of various infrared (IR) gas sensor technology applications in connection with carbon dioxide sensing, particularly as to measuring/controlling exhaust gas recirculation (EGR) to diesel engines.
To acquire desired information of any kind, measurements of physical parameters must be made. Devices that permit these measurements are broadly categorized as sensors. The term xe2x80x9csensorsxe2x80x9d encompasses a broad range of technologies and devices that respond to a physical stimulus (i.e. light, heat, sound, pressure, magnetism or a particular motion) and transmit a resulting impulse, generally for measurement or operating a control.
Sensors are widely used in many different applications. Some can be as simple as the direct measurement of a thermocouple, or as complex as all-weather imaging systems. Whatever the complexity of the sensor, an interaction between the sensor and its physical environment produces some kind of signal that ultimately leads to the desired information.
In many instances, sensor technology has become a basic enabling technology. The rapid increase in the interest in sensors has been driven by numerous applications, such as analysis of selected compounds in blood, in which sensors can provide a large public benefit.
In addition, sensors are of great importance in safety-related areas, with applications ranging from assessing the integrity of aircraft to fire safety monitoring. Common research and technology issues in these diverse applications include the interpretation of spectral signatures in terms of quantities of interest, such as concentrations, temperatures or thermal properties.
For example, market demand in gas measurement platforms for determining concentration levels of carbon dioxide, is driving increased activity in CO2 technology because in part of its utility in understanding and monitoring ventilation and Indoor Air Quality (IAQ). Building codes and standards governing ventilation in buildings, such as ASHRAE (American Society of Heating Air Conditioning and Refrigeration Engineers) Standard 62-99, have established minimum volumetric outside air requirements on a per-person basis.
Because individuals generally exhale a predictable amount of carbon dioxide, one application is to use this parameter to sense occupancy. An increasing or decreasing level of carbon dioxide can indicate ingress or egress of an indoor zone.
In addition, because outside levels are very low and constant, an indoor measurement can also provide a dynamic measure of the number of occupants of the space and the amount of low concentration outside air being introduced to dilute contaminant concentrations. As a result, a carbon dioxide measurement in a space can be used to measure or control per-person ventilation rates within a space.
Thus, while carbon dioxide is not a direct measure of indoor air quality, it has the potential to be an excellent measure of effective ventilation (mechanical ventilation plus infiltration). Generally, the higher the carbon dioxide concentration, the lower the ventilation. In other words, when indoor carbon dioxide levels are very high (i.e. above 1800 ppm) and ventilation is low (below 7 cfm/person), these conditions can allow contaminants to build up, causing irritation and discomfort.
Determining concentration levels of carbon dioxide is a powerful facility that can be useful in homes, office buildings, schools, and other commercial environments. However, implementable applications are limited by manufacturing and other costs, as well as health, safety, quality and other issues.
For example, in health and safety applications, oxygen sensors have been used to measure depletion of oxygen. However, oxygen sensors are not only expensive, but generally require periodic replacement or recalibration. Thus, it would be desirable to have an inexpensive alternative sensing method of measuring oxygen depletion.
In the automotive industry, there is also an increasing need for carbon dioxide sensor technology to improve the quality, safety and comfort of automobiles. For instance, it is known that the carbon dioxide concentration in the combustion air to an engine can be used to determine the amount of exhaust gases being re-introduced to the engine""s combustion air. This is because the carbon dioxide concentration of the engine exhaust is significantly higher than ambient air (i.e. 9 percent versus 350-550 ppm).
However, conventional sensing approaches for gases in engines utilize in-situ sensors that are directly exposed to the stream of gas being measured. Exposing these types of sensors to the harsh engine environment, particularly high temperatures, impairs sensing quality and results. Thus, it would be desirable to have an alternative sensing approach, to determine carbon dioxide concentrations, that could endure the harsh environment in the engine and still produce accurate measurements.
An equally important driver in the automotive industry is the incorporation of sensors into automotive products that aid in extending human life and improving safety. In one instance, there is a need to sense the presence of an individual within a vehicle""s trunk in order to prevent unwanted or accidental confinement that could lead to death.
In the area of sensor recalibration, there is an increasing need for sensors with an automatic calibration mode feature that has a fast recalibration time, and provides stable, false-free readings.
Accordingly, there is a need for an inexpensive sensor technology control approach that can be used as an indicia of concentration level characteristics of carbon dioxide. In addition, there is a need for a control approach that is suitable (i.e. standardizable) across a number of different applications.
The foregoing and other needs have been satisfied to a great extent by the present invention, which includes a very reliable method of determining concentration levels of gases, such as carbon dioxide.
More specifically, the present invention is achieved by use of a gas measurement criterion based on measuring the rate of change of carbon dioxide concentration and variations thereof, using optical methods. Optical methods are the most accurate and reliable method for measuring carbon dioxide because of its inert nature; carbon dioxide reacts poorly with other gases, and is difficult to measure reliably with sensors that depend on physical or chemical reactions.
In one aspect of the present invention, a method of measuring oxygen depletion is employed using the rate of change of carbon dioxide as a surrogate indicator of the amount of oxygen being depleted or displaced in the air. Depletion of oxygen can be measured in one of two instances.
In a first instance, if oxygen is being displaced by carbon dioxide, the natural consequence is that carbon dioxide will have to rise to very high levels to displace a significant amount of oxygen (e.g. greater than 30,000 ppm or 3 percent of CO2).
Conversely, and in a second instance, if oxygen is being displaced by another gas, then it follows that concentrations of carbon dioxide will ultimately begin to drop below normal atmospheric levels of 350-450 ppm. If the rate of fall of CO2 levels drops to below 300 ppm within 24 hours or less, for example, such a drop is reasonably indicative of oxygen depletion, and a warning or control is triggered.
More accurate control levels can be established given known space volume information. Also, the rate of change of carbon dioxide can be used if the rate of change exceeds normal rates that could be expected to be generated by human occupants.
In another aspect of the invention, a carbon dioxide sensor is remotely configured in an automotive or diesel engine in order to measure and control exhaust gas recirculation (EGR) to diesel engines. EGR techniques are used to reduce the emission of certain pollutants, such as nitrogen oxide (NOX), to meet EPA or other environmental requirements.
In order to maintain optimum operating conditions of the engine, and at the same time reduce emissions such as NOX, the ratio of exhaust gasses recirculated into the engine air intake to fresh outside air introduced to the engine, must be relatively constant. Maintaining this ratio can be difficult because of the varying operating speed and corresponding combustion air requirements of the engine.
This ratio may vary with engine design, but typically is approximately 20 to 25 percent EGR to fresh air. Since outside air has very low concentrations and engine exhaust has very high concentrations (i.e. 9-12 percent by volume), the carbon dioxide concentration in the mixed air compartment can provide an indication of the outside air to EGR air mixing, and can be used to maintain the appropriate EGR ratio. This approach employs a sampled method to determine carbon dioxide concentrations.
In this approach, a conductive sample tube is installed into the engine""s pre-combustion, air-mixing chamber and channels air to a remote CO2 sensor. This chamber is an area where exhaust gases are combined with ambient air before being introduced to the engine for combustion. This chamber may be inside the engine or as part of an assembly attached to the engine.
The conductive sample tube is of sufficient length to allow for additional cooling of the sample so that sampled gas temperatures are less than 50 degrees Centigrade, but not too long as to cause a significant delay in response time. Sampled air from the pre-combustion air-mixing chamber is pushed through by the pressure differential between the chamber (e.g. 1 atmosphere or more) and ambient pressures around the engine.
Optionally and alternatively, this sampling approach can facilitate control strategies for removing particulates, if necessary, that may require filtration. In addition, reducing the temperature of the sample allows for less complex and potentially more inexpensive CO2 sensor technology to be used, by eliminating the need for components and calibration that operate in high temperature environments.
Another feature and advantage of this aspect of the invention is presented if a conditioned sample is used, because it enables easier measurement of other gases, such as NOX, that are more easily measured using optical methods at temperatures below 50xc2x0 C.
In a third aspect of the present invention, the rate of change of carbon dioxide concentration levels is used to indicate the presence of an individual in an automobile trunk. There are several other sources of carbon dioxide that must be factored into consideration, since these sources may affect an accurate CO2 sensor reading. The two primary sources are: (1) leakage of carbon dioxide exhaust into the trunk compartment; and (2) deliberate injection of carbon dioxide into the trunk compartment to intentionally activate the sensor; perhaps to automatically open the trunk.
To guard against the occurrence of the two above-mentioned conditions, the CO2 sensor of the present invention and its warning/control logic is preferably configured to identify a rate of change of carbon dioxide that may be likely when one or more humans become trapped in a trunk.
Human carbon dioxide production is a function of activity and body size. Thus, an action/control point within the control logic is calculable based on rate of change data involving, for example, possible ranges of carbon dioxide production rates for assumed range or activity level(s). An action/control point within a control logic can also be calculated based on rate of change data, which calculations include, for example, the volume of the trunk; assumptions for air leakage rates for the trunk; and occupant ages.
These calculations preferably provide a carbon dioxide rate of change range in which it could be determined with reasonable accuracy that a human occupant is in the trunk. If the rate of change measured within the trunk falls within the calculated range, desired control or alarm strategies can be activated. Control or alarm strategies may include an indicator light, buzzer, opening of the trunk, flashing lights, sounding horn, and the like. If suspicious CO2 levels were detected outside the desired or predetermined sensing range, an alternative indicator can be activated.
In a fourth aspect of the present invention, an automatic calibration mode for carbon dioxide sensors is provided. Calibration is based on using a zero gas for calibration. This approach can be used for other infrared sensors, besides carbon dioxide sensors, as well.
Given that ambient levels of carbon dioxide are generally 350 ppm or higher, and that carbon dioxide levels tend to change gradually within a space, the CO2 sensor of this aspect of the invention is designed to recognize a distinctive rate of change pattern of carbon dioxide concentrations that would be indicative of a zero calibration routine. Once this distinctive pattern is identified, the sensor is triggered to go into a calibration mode and reset its calibration based on the CO2 concentrations being measured during the calibration mode.
In a preferred embodiment, the distinctive pattern may comprise a dramatic drop of 200 to 300 ppm or more over approximately 15 to 30 seconds in the sensor reading. Another distinctive pattern may comprise a stable reading for the subsequent 30 seconds, indicating a constant flow of the calibration gas to the sensor. This pattern may optionally activate the calibration mode of the sensor. Provided that carbon dioxide concentration levels remained stable through the calibration period (e.g. 1 to 5 minutes), the sensor may recalibrate itself to zero based on the same gas it is measuring.
There has thus been outlined rather broadly the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein as well as the abstract included below are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.